Frequency selective device



Oct. 20, 1925. 1,557,562

G. CRISSON FREQUENCY SELECTIVE DEVI CE Filed Oct. 16. 1919 //f/ M "Q 14 l 15V V EN TOR. 63 622195014- g ATTORNE GEORGE GRISSON, 01F HOBOKEN, NEW JERSEY, ASSIGNO R TO AMERICAN AND TELEGRAPH COMPANY, A (JUERPO.htui'lll'ON OFATEW YUR raneunlvov snnnccrrvn neuron.

Application filed Gctaner 16, 1919. Serial No. 331,085..

.To all whom it may concern;

Be it known that LGEoRoE'CRIssoN, resid ing at Hoboken, in the county of I-ludson and State of New Jersey, have invented certain Improvements in Frequency Selective Devices, of which the following isa specification.

This in vention relates to frequency selecciated with telephone apparatus, it being readily understood, however, that the invention is applicable to many other branches,

of the arts. The device as described herein .is used for preventing disturbances of lOW frequencv, as frequently exist'ontelephone lines dueto telegraphic and-other signaling currents, from causing noise in areceiver,

the device being so arranged as to have 'no eifect on the voice currents, which are of higher frequency than the disturbances.

One form and arrangement of apparatus and circuits embodying the invention 1sv shown in diagrammatic view in Figure 1 of the accompanying drawing, F ig, 2 being a graph showing the characteristics ,of the arrangement of F 1g. 1 for var ous values of certain elements of the arrangement.

In the form shown in Figure 1, the.inven tion is especially suitable for use as a high impedance monitoring circuit forserviceobserving and other necessary purposes.

Reference letter T of Fig. 1 designates a telephone line and numeral 7 a telephone receiver, which is associated with the line in intensity to voice currents, in the line but very weakly if at all to currents of low free quencies, such as disturbances caused by telegraphic or other signalingcurrents. The apparatus which associates the receiver with the line comprises a transformer 1, the pri- -mary of'which is connected across the telephone line in series ,with a resistance 2 and a condenser 3.- The secondary winding of the transformer is associated with the grid or input circuit of an electron tube amplifier 8, the output circuit of which is associated by means of the transformer 4 with the receiverj 7. The transformer 1- behaves substantially like an inductance coil,

its secondary circuit being effectively open because of the high impe a circuit of thetransformer has therefore sub- TELEPHCNZE nce ofthe grid I fl of the electron tube. The primary stantially the same characteristic as a circuit comprised of seriallyeonnected'inductance,

resistance and capacity elements,- and the amplifier causes a flow of currentthrough the receiver proportional to the drop of .vo'lt-' again the inductance element of the circuit. \Vith a'high inductance in the transformer 1, the loss to transmission over the line T and the effect on the balance of nearby reeaters can be made as small as we please.

buch an inductance alone might tend-to per j mit low frequency impulsesto reach the receiver 7. Placing the resistance2- in series with the transformer 1 reduces the efiiciency of the system for low frequencies, but at-the same time this keeps the impedance high across the line T. This avoids harmful transmission losses and impedance effects in the line that would, becaused if the discr mination against low frequencies were obtamed by reducing the inductance of the transformer 1. The capacity of the-con 'frequencies, and by proper proportio'nment of theelements of the circuit, as explained below, this dropmay be made substantiallyequal (in absolute value) to the voltage across the l ne for currents-of voice frequensuch manner that it responds with normal' cies, and considerably less than the voltage...

across the line for currents of low frequencies so that the receiver responds with norbeing determined at anyfrequency by the ratioof the voltage across the line T to the voltage across the inductance element of the circuit. In telephony it has been found con,- venient to express transmission loss interms of the number of miles of standard cable which has an equal loss in transmission at 800 cycles, the length M in miles of a standard cable which will cause a ratio I V l of output to input voltage being well known equation:

1 w =21rf =j 1 3 when w=l the circuit is resonant.

E:E. M. acting in the circuit e:drop of voltage in the inductance 7 element.

1 j 1 e jwL rhenv=+ i (i r. n+ ticto simplify this expression,

Let r= (OCL inductive reactance at resonant frequency.

then R w ll (2) ratio of resistance to co L also to =.ww

Substituting 2 a and 4 in (1 and simplifying e +i' V= i 2 (5) and the absolute value of V is this equation having a negative "value when there is a loss in transmission and a positive value when there is a gain, i. e. if the voltage across the inductance element is larger than the voltage across the line.

This equation is plotted for several Values of r in Fig. 2, the ordinates of this graph Hence given by the being in terms of miles gain or loss and the abscissae in terms of w. The curves for which 1 is equal to or greater than unity show a rapidly increasing loss ofv transmission for low values of w, i e. at low frequencies, and a substantially constant transmission, with but slight gain or loss, at high frequencies. By proper choice of the elements of the circuit, therefore, the arrangement may be designed to cause a decided loss intransmission for low frequency disturbances without having any appreciable effect in the transmission of the voice currents. For example if the induc-v Y ta'nce of transformer 1 is 5 .henrys and if the requirements are that the transmission of. 100 cycle currents must be reduced by 14 miles and the transmission of the voice currents'must not be increased at any frequency more than 1 mile, it will be found that the curve for which 1 :1.1 gives the sharpest cut-oil consistent with the requirements. This curveindicates a loss of 14 miles at 00:3 .5, which then corresponds to 100 cycles, The resonant frequency is therefore:

j, 45 =222 cycles,

from which w :21tf :1395,

1 G fELI'tLClS mf.

and R m L:7673 ohms.

From this value of it must be substracted the eflective resistance of the transformer and the resistance of the source if accurate results are desired.

If, then, the above determined values of L, R and C are used in the arrangement of.

this invention, it willbe found to have the characteristics determmed by the curve for which r=l.l. Its transmission at low fre quencies is exceedingly poor and it does not reach the point of zero loss until 20 is equal to 1.13 or 244 cycles. For currents above this frequency the curve shows a slight gain, which, however, gradually declines as the frequency rises, (the curve being tangent to the axis of abscissae when w is infinite).

A smaller value of 0* gives a sharper cut- .ting oil action but a corresponding rise of the maximum gain. A- larger value of r,- on the other hand,v gives a more gradual cut-off with a smaller rise of the maximum gain. The best value of 1' depends, of course, on the actual condition to be met.

The dotted curve is drawn to show the characteristic of the arrangement in the special case when the value of C is infinite, in other words, when the condenser is omitted. In this case there is no resonant frequency and w becomes the frequency at which the A resistance and reactance are equal, 1" being chosen as unity. In this case .121; t R+jwL w i |V| am. As shown by the curve, the characteristic of the arrangement without a condenser has only a radual cut-ofi which may be des1rable un er certain conditions.

What is claimed is: 1. A- device formonitoring on a telephone transmission line, comprising a telephone receiver, a three electrode vacuum tube with its output circuit associated with said re ceiver, a shunt across the line having high impedance compared to the characteristic impedance of t-he'line, comprising inane: lance resistance and capacity elements, and means connecting the grid of said tube in shunt relation to said inductance element;-

2. A device for monitoring on a transmission line, comprising a three electrode vacuum tube, a. bridge across the line having high impedance compared tothe characteristic impedance of the line, and a connection between said bridge and the grid of said tube, the impedance elements of the bridge and the connection of the grid being so related as to give a high loss on the grid for frequencies below a certain limit anda lowuniform ain or loss for frequencies above that limit.

3. In combination, a transmission line, a telephone receiver to be connected therewith at an intermediate point, a shunt across the line comprising inductance, re-' sistance and capacity "elementsinseries, a

three-electrode vacuum tube with its grid circuit connected to impress upon the grid the voltage drop through the said inductance, said telephone receiver being con-..

ing, a three-electrode vacuum tube compris-Y ing the corresponding transformer secondary winding in its gridcircuit, said telephone receiver being connected to the plate circuit of said tube.

5. In combination, a transmission line having'a certain characteristic impedance, a

telephone receiver to be connected therewith at an intermediate point, a shunt across the line having an impedance high compared to the said characteristic impedance of the line and comprisinginductance, resistance and capacity elements in series, a threeelectrodc vacuum tube with its grid circuit connectedtorimpressupon thegrid the volt-- age drop; through. the said inductance,- said telephone receiver bein connected with the plate circuit or said tu e.

6; In combination, a transmission line, a

telephone receiver to be connected therewith at an intermediate point.- a shunt across the line comprising inductance, resistance and capacity elements inseries, said inductance and capacity elements being tunedttra low frequency. compared to the essential voice frequency range, a three-electrode vacuum tube with its grid circuit connected to" impress upon the grid the voltage drop through the said inductance, said telephone receiver being connected with the plate circuit of said tube. i

- In testimony whereof, I, have-signed my name to this specification this 15th day of October 1919.v

- GEORGE GRISS ON. 

